The call-waiting (CW) telephony feature is generally well-known and has been offered in the last few years by most telephone operating companies. In conventional CW, when a customer who subscribes to CW service is engaged in a telephone conversation with a second party and a third party calls that customer, the customer receives a special call-waiting tone, typically 440 Hz for 300 milliseconds, whereas the third party receives audible ringing. If the CW customer chooses to receive the call from the third party, a switch-hook flash holds the second party while the subscriber's connection is switched to the third party. If the first call-waiting tone is ignored, a second CW tone is given after a few seconds and if this signal is also ignored, the third party is given a busy tone.
Early versions of call-waiting service are discussed in U.S. Pat. No. 3,133,995 to Zarouni whereas later versions are discussed in Pommerening et al., U.S. Pat. No. 3,963,874, George et al., U.S. Pat. No. 3,997,731, Beth, U.S. Pat. No. 3,584,156, and Brecher, U.S. Pat. No. 4,661,975.
When a call-waiting subscriber receives the call-waiting tone(s) he is frequently very reluctant to interrupt the existing conversation for what may be an entirely unsolicited and unwanted call from a third party. However, the CW subscriber would be in a position to make an informed decision if he knew the identity of the third party.
Another customer service presently being introduced to the marketplace is calling number identification (CNID). Early proposed versions of this service may be found in U.S. Pat. Nos. 3,727,003 and 3,812,296 to Paraskevakos, as well as in U.S. Pat. No. 3,787,626 to Subieta. A more recent scheme of CNID is discussed in U.S. Pat. Re. 31,789 to Hashimoto. In these patents, the directory number of a calling party is coded as digital pulse trains and is transmitted from the originating central office to the terminating office and sent to the called party's telephone set either before it is sent an alerting signal such as ringing or during the silent interval between the first and second ringing signals. In these patents, the called party's telephone set must be equipped with receiving circuitry for decoding the digital pulse trains as well as a display device to indicate the identity of the calling party. The called party is then placed in the position of being given the opportunity to answer the call or ignore it.
The contemporary method and protocols of data transfers necessary to the realization of the CNID feature are available from Bellcore Technical References TR-TSY-00030 and TR-TSY-00031. Basically, these specifications require the use of modem data (frequency shift keying - FSK) to transfer the identification information from the central office to the subscriber set during the silent interval between the first and second ringing signals.
Yet another patent related to calling number identification is U.S. Pat. No. 4,873,719 to Reese. This patent is directed to the privacy issue of CNID and proposes to solve the question by granting the calling party exclusive control of the delivery of his identity to the called party. The patent proposes to use conventional call-waiting tones and to deliver the calling party identification information using Dual-Tone Multifrequency (DTMF) data signals while the called party is off-hook.
A first difficulty with this approach is that detection of a conventional call waiting tone in the presence of voice is unreliable at best. Secondly, the appearance of modem data signals (frequency shift keyed data - FSK) or DTMF encoded data signals for a duration of even a few seconds is very annoying to the conversing parties and is thus undesirable.
It is therefore an object of this invention to provide an improved call waiting service to telephone subscribers.
It is also an object of this invention to provide an improved method and apparatus of providing calling party identification information to a called party's telephone set while the latter is off-hook.
It is another object of the invention to provide a robust alerting scheme for use on in-use subscriber loops.
It is yet another object of the invention to provide CNID on a busy telephone line only when the called party's subscriber telephone set is capable of receiving and displaying the information.
It is a further object of the invention to provide this improved call waiting service on analog subscriber lines using non-ISDN (Integrated Services Digital Network) technology.